1. Field of the Invention
Embodiments of the present invention generally relate to an electrical submersible pump assembly adapted to efficiently reduce a gas content of a pumped fluid. Particularly, embodiments of the present invention relate to an electrical submersible pump assembly having a device to direct gas flow leaving the assembly.
2. Description of the Related Art
Many hydrocarbon wells are unable to produce at commercially viable levels without assistance in lifting formation fluids to the earth's surface. In some instances, high fluid viscosity inhibits fluid flow to the surface. More commonly, formation pressure is inadequate to drive fluids upward in the wellbore. In the case of deeper wells, extraordinary hydrostatic head acts downwardly against the formation, thereby inhibiting the unassisted flow of production fluid to the surface.
In most cases, an underground pump is used to urge fluids to the surface. Typically, the pump is installed in the lower portion of the wellbore. Electrical submersible pumps are often installed in the wellbore to drive wellbore fluids to the surface.
In a well that has a high volume of gas, a gas separator may be included in the ESP system to separate the gas from the liquid. The gas is separated in a mechanical or static separator and is vented to the well bore where it is vented from the well annulus. The separated liquid enters the centrifugal pump where it is pumped to the surface via the production tubing.
In a well that produces methane gas, the electrical submersible pump is generally used to pump the water out of the wellbore to maintain the flow of methane gas. Typically, the water is pumped up a delivery pipe, while the methane gas flows up the annulus between the delivery pipe and the wellbore. However, it is inevitable that some of the methane gas entrained in the water will be pumped by the pump. Wells that are particularly “gassy” may experience a significant amount of the methane gas being pumped up the delivery pipe.
For coal bed methane wells, it is generally desirable that no methane remain in the water. Methane that remains in the water must be separated at the surface which is a costly process. Therefore, a gas separator may be used to separate the gas from liquid to reduce the amount of methane gas in the pumped water.
FIG. 1 shows a prior art downhole electric submersible pump (ESP) assembly 10 positioned in a wellbore 5. The ESP assembly 10 includes a motor 20, a motor seal 25, a gas separator 30, and a pump 40. The gas separator 30 is positioned between the pump 40 and the motor seal 25. The motor 20 is adapted to drive the gas separator 30 and the pump 40. A central shaft extends from the motor 20 and through the motor seal 25 for engaging a central shaft of the separator 30 and a central shaft of the pump 40. Fluid enters the ESP assembly 10 through the intake port 32 in the lower end of the gas separator 30. The fluid is separated by an internal rotating member with blades attached to the shaft of the gas separator 30. The gas separator 30 may also have an inducer pump or auger at its lower end to aid in lifting the fluid to the blades. Centrifugal force created by the rotating separator member causes denser fluid (i.e. fluid having more liquid content) to move toward the outer wall of the gas separator 30. The fluid mixture then travels to the upper end of gas separator 30 toward a flow divider in the gas separator. The flow divider is adapted to allow the denser fluid to flow toward the pump, while diverting the less dense fluid to the exit ports 38 of the gas separator 30. Gas leaving the gas separator 30 travels up the annulus 7.
One problem that arises is that the gas leaving the gas separator may commingle with the fluid flowing toward the intake port. In this respect, the gas content of the pumped fluid may be inadvertently increased by the gas leaving the separator. The increase in gas entering the gas separator when this occurs reduces the efficiency of the gas separator which may result in incomplete separation of the gas from the liquid. This has negative effects on pump performance and in a coal bed methane well will result in methane in the water being pumped from the well.
There is a need, therefore, for an apparatus and method for efficiently reducing a gas content of a pumped fluid. There is also a need for apparatus and method for maintaining a separated gas from a fluid to be pumped.